A conventional electroluminescent (EL) lamp has a substantially transparent or transmissive substrate, a first transmissive electrode on the substrate, a back or second electrode opposite to the transmissive electrode, and an intermediate layer between the first and the second electrodes. The intermediate layer includes a dielectric layer and a means for producing light emission. The means for producing light emission is an electroluminescent phosphor.
With this structure, the electroluminescent lamp is luminous when an alternating electric voltage is supplied between the first and the second electrodes. Light emission occurs because electrons are excited to a conduction band by the electric field and are accelerated to activate luminescent centers in the phosphor from a ground state. Such activated luminescent centers emit light when they return to the ground state.
The dielectric layer included in the intermediate layer serves to increase the strength of the electric field developed in the electroluminescent layer. In addition, the dielectric layer is useful in raising the breakdown voltage of the electroluminescent lamp.
The means for increasing the strength of the electric field in the electroluminescent layer includes barium titanate, barium strontium titanate, yttrium oxide, aluminum oxide, silicon nitride, silicon oxynitride, tantalum pentoxide, lead titanate, and similar equivalents. These substances have varying characteristics of dielectric constant, moisture resistance, adhesion to other layers in the lamp, and optical refractive index.
A further desirable characteristic for an EL lamp is the transparency of the electrode in the desired light-emitting side of the lamp. Even more desirable for many applications would be a highly transmissive lamp, where the dielectric layer and phosphor layer were also highly transmissive, something so far not achieved because of the high refractive index of many such materials and the scattering of light in compositions. Double-sided lamps have been built that emit light from both sides, but which are not themselves transmissive to light. What is needed is a lamp that is highly transmissive all the way through, whether energized or de-energized.